At present, a reinforcing member for a carcass constituting a skeleton of a radial tire for a passenger vehicle, in particular, a belt which is generally used as a reinforcing member for a crown portion of a carcass is mainly constituted such that two or more steel belt layers composed of layers in which rubberized steel cord obliquely arranged with respect to the tire equatorial plane are used, and that the steel cords in the belt layers are crossed each other.
In recent years, the importance of environmental performance is increasing, and there is a growing need for lightweight of a rubber article or a tire in which a steel cord is employed as a reinforcing member. Examples of a method in which the weight of a tire is reduced include a method in which the amount of rubber used in a belt treat is reduced and the thickness of the belt is reduced. However, when the amount of rubber used in a belt treat is reduced, the distance between cords of a first belt layer and a second belt layer becomes short, a so-called belt edge separation (BES) in which rubber separation originating from the cord end at the end portion in the belt width direction easily propagates between cords easily occurs, and the durability is reduced. As a method of improving such BES, a method is known in which the thickness of the rubber at the belt end portion is increased compared with a normal tire. Of course, however, the weight of the tire is thus increased, which is contrary to the lightweight of the tire which is an expected object.
Examples of a method of reducing the weight of a tire other than reducing the amount of rubber used for a belt treat include a method of reducing the amount of steel used in the tire, for example, a method of reducing the end count of steel cords is conceivable. However, when the end count of steel cords is reduced, the rigidity of the belt is reduced, which is not preferred. Under such circumstances, many proposals are being made regarding the improvement of the lightweight or durability of a tire. For example, Patent Document 1 proposes a steel cord having an M(M=2 to 5)+N(N=1 to 3) structure and the numbers of filaments thereof satisfies N≧M for the purpose of reducing the weight of a tire. Patent Document 2 discloses a steel cord having a 2+3 structure for the purpose of improving the durability of a belt. Other than these, Patent Documents 3 to 7 disclose a steel cord having a 2+3 structure for the purpose of improving a variety of physical properties or workability of a reinforcing member of a tire which are needed.